1. Field of the Invention
The present invention relates to a driver circuit of a light-emitting element.
2. Description of the Related Art
With the development of a light-emitting element such as a light-emitting diode, a driver circuit has been conceived which drives a light source which emits light and in which a plurality of light-emitting elements are connected in series.
As shown in FIG. 8, a driver circuit 100 of related art comprises a rectifying bridge 10, a controller 12, a current controlling inductor 14, a transistor 16, and a regenerative diode 18.
The current controlling inductor 14 is connected in series to a series-connected structure 102 of a plurality of light-emitting elements to be driven. When the light-emitting element is the light-emitting diode (LED), the light-emitting diodes are connected in series with the forward directions aligned with each other. The regenerative diode 18 is connected in parallel to the current controlling inductor 14 and the series-connected structure 102 of light emitting elements. The transistor 16 is connected in series to the current controlling inductor 14 and the series-connected structure 102 of light-emitting elements, to control a current flowing in the light-emitting element included in the LED-connected section 102 and the inductor.
A power supply voltage is applied from the rectifying bridge 10 to the series-connected structure 102 of the light-emitting elements and the current controlling inductor 14. When the controller 12 sets a gate voltage of the transistor 16 to a high level, the transistor 16 is set in the ON state. With this process, as shown in FIG. 9, a voltage Vds between the drain and source of the transistor 16 is reduced, and, because of the current limiting action of the current controlling inductor 14, the current ID flowing through the series-connected structure 102 of light-emitting elements and the current controlling inductor 14 gradually increases. The controller 12 switches the gate voltage of the transistor 16 to a low level before the current ID exceeds a rated value of each light-emitting element. With this process, the transistor 16 is set to the OFF state, the voltage Vds between the drain and source of the transistor 16 becomes the power supply voltage Vdc, and the current ID is reduced. The energy stored in the current controlling inductor 14 during the OFF state is regenerated to the power supply by a regenerative current Ioff via the regenerative diode 18. In this manner, by controlling switching of the transistor 16 by the controller 12, it is possible to emit light while maintaining the current of light-emitting element to be less than or equal to the rated value by the function of the current controlling inductor 14.
In the driver circuit 100 of the related art, a full-wave rectified power supply voltage is supplied from the rectifying bridge 10. In this process, as shown in FIG. 10A, in a region A in which the power supply voltage is low, there has been a possibility of abnormal oscillation of the switching of the transistor 16.
In consideration of this, as shown in FIG. 11, a capacitor C for averaging the power supply voltage may be connected on an output side of the rectifying bridge 10, to apply a measure to average the power supply voltage, as shown in FIG. 10B. However, with the provision of the capacitor C, another problem is caused in which a power factor of the driver circuit 100 is reduced.
In addition, in the driver circuit 100 of the related art, when the controller 12 switches the transistor 16 from the OFF state to the ON state, the voltage applied to the series-connected structure 102 of light-emitting elements, the current controlling inductor 14, and the transistor 16 rapidly changes. Because of this, as shown in FIG. 12, spike-like noise may occur at a rising portion of the current ID.
Moreover, in the driver circuit 100 of related art, a power supply is supplied from the rectifying bridge 10 through a limiting resistor Rstart to the controller 12. In such a structure of the driver circuit 100, even when the number of series connections of the light-emitting elements is increased and the power supply voltage Vds is increased, the power supply voltage supplied to the controller 12 is still maintained at a low value. As a result, the electric power consumed by the limiting resistor Rstart is increased and efficiency of the overall circuit is reduced.
For example, when a commercial AC power supply of 100 V is rectified by the rectifying bridge 10 and a power supply voltage Vdc of approximately 120 V is to be supplied, if the number of connections of the light-emitting elements is increased and light-emitting elements of a few tens of watts (W) to a few hundred watts (W) are to be driven, the controller 12 must drive the transistor 16 with a current capacity of approximately 100 mA. In such a state, electric power of approximately 10 W would be consumed at the limiting resistor Rstart, and the overall power efficiency of the driver circuit 100 is reduced.